This application claims the benefit of U.S. Provisional Application Ser. No. 61/069,683 filed Mar. 17, 2008, the disclosure of which is hereby incorporated herein by this reference.
This disclosure relates to current sensing devices and more particularly to current sensing devices that self calibrate upon receipt of a calibration signal.
Many electrical or electronic devices require that the current input or output be sensed and/or controlled for proper operation. Among the electrical applications wherein current sensing is important are applications in which a motor is utilized.
One previous method of monitoring a motor for over current, correct current, or under current conditions, involved installing a current sensor on the load wires feeding the motor. When a desired motor current condition occurred, the current sensors supplied a binary or analog output that could be used by another circuit or another device to simply monitor the condition or to take some action based upon the output. An analog output from a current sensor could be used by another device to determine current conditions as mentioned above. A binary current may be used to monitor two separate conditions. Unlike analog outputs, the binary output of the current sensor relies upon a threshold, either fixed or adjustable, in order to discern between separate conditions.
Analog output current sensors are typically more expensive than binary output current sensors, they rely on more expensive other circuits or devices to interpret their output and therefore may be less desirable.
Binary output current sensors with fixed thresholds are less desirable than adjustable threshold current sensors if the correct current condition of a motor is not near the same level as the fixed threshold. One drawback of fixed threshold binary output current sensors is that when the correct current condition of a motor is not near the same level as the fixed threshold, a simple yes/no condition is all that can be utilized. However, over current, correct current, and under current conditions can be indicated by the binary output of a fixed threshold current sensor if the threshold of the current sensor is near the correct current condition of a motor. Therefore, in order to obtain an indication of over current, under current, and correct current conditions, utilizing fixed threshold current sensors, it would be necessary to be order specific fixed threshold current sensors with the appropriate threshold for individual applications which might require users to carry a large inventory of fixed threshold current sensors with varying threshold values.
Binary output current sensors with adjustable thresholds can be adjusted to almost any level within the range of the current sensor making them very universal. One current sensor could be used for varying current sized motors. A yes/no condition or over current, correct current, and under current conditions can be utilized with the adjustable current sensor.
When monitoring a motor, it may be necessary to engage a sensing device such as a adjustable threshold binary output current sensor in proximity of the current carrying wires feeding the motor. Typically the motor starter box is the best place to access the current carrying wires and for installing the current sensing device. The National Electric Codes (the US code book for electrical installations) requires a motor starter box for most applications utilizing motors. The motor starter box typically includes a relay, over current protection, and other devices or circuits all of which are enclosed in a housing to protect outside agencies from high voltage or shock hazard conditions.
There is typically room within such a motor starter box to install a current sensing device. That device, once installed, is within a high voltage or shock hazard environment. Some existing designs of current sensors require either an unsafe code violation, or an inconvenient removal of the device from the motor control box while power is off, powering the device some other way, pressing a reset button, powering down, putting the device back in the cabinet and closing the door before powering up the motor in order to calibrate the device.
The National Electric Code prohibits having a motor control cabinet door open while the motor is running. Existing sensing products require that the motor controller cabinet be open to allow access to the sensing device in order to initiate a re-calibration sequence (IE: press a button).
One example of a prior art current sensor for a motor application is a Functional Devices, Inc. model RIBXGTA. This device can easily be clipped around or near the current carrying wire(s) within a motor starter box. The RIBXGTA current sensor includes an accessible potentiometer intended to be adjusted while the motor is running to allow manual setting of its threshold. The threshold can be set at a level that would provide a yes/no output or at a level that the output could represent over current, correct current, or under current condition. Placement of the RIBXGTA current sensor in the motor starter box mandates that the cover of the motor starter box be open while the motor is powered up and operating in order to access the adjustment device and set the appropriate threshold. Doing so with the cover open exposes one to high voltage, creates a shock hazard and is not allowed by the National Electrical Code.
Another example of a current sensor for motor applications is Functional Devices, Inc. model RIBXLRA configured for placement of the current sensing device within the motor starter box, while locating the mechanism to make adjustments to the threshold in a separate device that is mounted to the outside of the motor starter box. Such an arrangement works around the flaw of the aforementioned Functional Devices, Inc. model RIBXGTA in that the motor may be running with the motor starter box closed while an adjustment is made to the threshold.
Other current sensors exist that work around the problem of having to adjust a threshold in a shock hazard environment by having the adjustable current sensor self calibrate. One such prior art current sensor is Veris model H10F. During calibration, power must be applied to the H10F current sensor then a button must be pressed to tell the unit to go thru a self calibration mode. One disadvantage of the H10F current sensor is that it must be powered up, either before installing the unit in the motor starter, or after installing the unit in the motor starter. The inconvenience of having to have the H10F current sensor out of the motor starter box, powering it up, pushing the button, then installing the unit is inherent to such a device. The only other option is to install the H10F current sensor in the motor starter box, leave the motor starter box open, power up the unit, then push the button in a shock hazard condition. Again the latter option does not comply with the National Electrical Code standards.
The disclosed self-calibrating current sensor accepts a binary input from a source without creating a shock hazard for an operator thereby addressing some shortcomings of prior devices.
The disclosed self-calibrating current sensor accepts and stores a command to re-calibrate while in a safe, powered down mode. A user may in one embodiment flip a switch on the device while it is still inside the control cabinet with power turned off. In another embodiment a self-calibrating current sensor accepts a binary change from an outside source, such as a controller electrically coupled to the device, and begins a recalibration process upon a change in state of the binary signal received.
In one embodiment of the disclosed self-calibrating current sensor, the device accepts and remembers a command to initiate a calibration sequence while in a safe, powered down condition. Upon power up, the device will recognize the command, wait a prescribed time period then automatically re-calibrate. In another embodiment of the disclosed self-calibrating current sensor, the device accepts a command in the potential shock hazard powered up state without exposing an operator to such a potential shock hazard by accepting a binary input from a device such as a controller.
According to one aspect of the disclosure a self-calibrating current sensor for sensing the state of the current passing through a load line enclosed within a housing comprises a sensor, a binary input circuit, and a controller circuit. The sensor is disposed adjacent the load line for sensing a current passing through the load line and has an output at which a signal indicative of the level of current passing through the load line is present. The sensor is positioned within the housing. The binary input circuit is configured to generate at least one binary signal and is configured to change the level of the at least one binary signal without the need of opening the housing. The controller circuit has a first input coupled to the output of the sensor for receiving the signal indicative of the level passing through the supply line and a second input coupled to the binary input circuit for receiving the at least one binary signal. The controller is configured to provide a signal indicative of the status of the current in the load line on an output.
According to another aspect of the disclosure, a method of sensing a current flowing through a load wire disposed at least in part within an housing includes a providing a self calibrating current sensor step, a positioning a sensor component step, a powering up the self-calibrating current sensor step, a generating an initiate calibration signal step, a calibrating the self-calibrating current sensor step, a comparing the signal indicative of the level of current in the load line to the stored normal operating current step and a generating a fault signal step. The provided self-calibrating current sensor includes an input circuit for providing an initiate calibration signal to initiate calibration of the self-calibrating current sensor, a sensor component coupled to the load line in a manner to allow the sensor to generate a signal indicative of the level of the current in the load line and a controller for receiving the signal indicative of the level of the current in the load line and storing the same and for receiving the initiate calibration signal. The positioned sensor component of the provided self-calibrating current sensor is positioned within the housing for sensing the current flowing through the load wire. The initiate calibration signal is generated without opening the housing. The calibrating the self-calibrating current sensor step includes storing a normal operating current following generation of the calibration signal. The comparing the signal indicative of the level of current in the load line to the stored normal operating current step is performed following the calibrating step. The generated fault signal is available without opening the housing and is generated when the signal indicative of the level of current in the load line is outside a selected range of the stored normal operating current.
Additional features and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of a preferred embodiment exemplifying the best mode of carrying out the invention as presently perceived.